he most dramatic ways in which we experience Earth's climate -- monsoons, drought, cold snaps, and storms -- come from the sky. But climate is as much a product of the sea as the sky: whatever happens to one affects the other.

In the equatorial waters of the Pacific, the interplay of sea and sky produce a dramatic climate fluctuation known as El Niño. Fisherman on the coast of Ecuador coined the term, which means "the child," a reference to the infant Jesus. In some years, a warm current appears off their shores and the fishing turns bad. This typically becomes noticeable in the Christmas season, thus the reference to the coming of Jesus. Sometimes, the warm current persists well into the spring. And it does more than make South American fisherman go hungry; it can also affect global weather patterns.

The El Niño of 1997-98 brought drought and forest fire to Indonesia. An ocean away, heavy precipitation triggered floods and mudslides to California. These extremes of weather trace to a chain of events that unfolded in the equatorial waters of the Pacific Ocean over a period of months. Though scientists recognized elements of the El Niño phenomenon as far back as the 1920s, it is only recently that the connections between them and details of the process have been revealed.

El Niño mudslides swept away all but the skeleton of this house in Southern California.

El Niño begins with a slackening of easterly winds along the equator. Normally, the trade winds blow west in the tropical Pacific. This causes surface water to pile up; in fact, the sea surface is about a half meter higher in Indonesia than off South America. But for some reason -- nobody knows why -- the trade winds slacken. As a result, the warm surface water sloshes eastward.

The effects then ripple across the globe. Rain storms -- fueled by moist air rising from the ocean surface -- follow the warm pool eastward. This shift of storms affects weather systems far away from the tropics. To understand how this happens, it helps to imagine the atmosphere as a river of air. Where a river flows over a submerged boulder, a series of waves form downstream on the water's surface. In the same way, waves form in the atmosphere when air blows over a meteorological obstacle -- in this case, dense tropical rain clouds. So when El Niño shifts the clouds eastward, the pattern of crests and troughs in the atmosphere also shifts. These waves can affect the tracks that storms take across the continents, and also the trajectories of high-altitude winds called jet streams.

The effects of these disturbances can reach thousands of miles. Tropical storms may be steered to regions they normally don't visit. Monsoon rains may shift away from Australia and Indonesia, causing disastrous drought and forest fires. The southern tier of the United States may experience higher-than-average rainfall and western Canada may have a mild winter. But "may" is the key word: El Niño's effects are complex, and can't be predicted precisely.

What scientists are getting a lot better at, however, is anticipating the arrival of The Child. The last El Niño, in 1997-98, was predicted several months ahead of time by combining computer models with careful observations of changes in sea surface temperature and atmospheric conditions. The predictions can help farmers and other people affected by El Niño to prepare. In Peru, for example, farmers and government planners decide what crops to grow based on the expected climatic conditions. In an El Niño season, when rainfall is generally heavier, they can choose to grow more rice -- a plant that thrives on moisture -- and less cotton, which needs a drier climate. Countries afflicted by El Niño drought, in contrast, can plan to conserve water and try to prevent brush fires. We will never be able to prevent El Niño, but at least we can be prepared for it when it comes.

The Great Ocean Conveyor Belt

l Niño has strong effects on weather and climate measured on a scale of months. But broad circulation patterns in the world's oceans also control climate patterns on a scale of decades to centuries. In fact, Europeans may owe their very existence to heat transported to the North Atlantic by a global system of currents known as the Great Ocean Conveyor Belt.

Unlike most ocean currents, which are driven by surface winds, the flow of the Conveyor Belt is driven by differences in the temperature and salinity of seawater. Cold, salty water sinks; warm, less salty water stays on the surface. This sets up a system of interconnected deep and shallow currents that transport heat from the tropics to higher latitudes. The Gulf Stream, for instance, is part of the Conveyor Belt. It helps to ferry heat to the North Atlantic that keeps Europe's winters relatively mild. Without the Conveyor Belt, the city of Dublin would have an Arctic climate and much of the rest of Europe would be too frigid for trees to grow.

The Conveyor Belt begins in the North Atlantic. There the water grows colder and saltier and sinks. This feeds a sluggish mass called the North Atlantic Deep Water. With a flow 20 times that of all the world's rivers combined, the deep water flows southward over the ocean floor. The flow is vast, but not swift: Once deep water forms, it may not touch the open air again for 1000 years.

Some scientists are concerned that polar ice will melt, with potentially disastrous effects across the globe.

The flow surfaces again around Antarctica, where it is chilled again. Ultimately, it surfaces in the Indian and Pacific Oceans and then flows back north into the South Atlantic and eventually into the North Atlantic. The heat it carries warms the frigid, eastward moving air masses from Canada, Greenland, and Iceland. This flow of ocean-warmed air helps keep the winter climate of Europe balmier than at equivalent latitudes in North America.

The Great Conveyor Crash?

he Great Ocean Conveyor Belt is mighty, but it is not invincible. Some scientists believe that it may be vulnerable to the effects of global climate change, whether natural or manmade. Computer simulations suggest that if the North Atlantic warmed just slightly, or were diluted by freshwater from increased rainfall or melting continental glaciers, the formation of deep water might decrease enough to shut down the conveyor. Both effects -- warming or dilution -- are possible consequences of global warming.

Global average temperatures are now on the rise. Scientists are still struggling to figure out how much of the rise is natural -- part of the ceaseless cycles of climate on Earth -- and how much is the result of the burning of fossil fuels and other human activities. Either way, scientists are closely watching the polar regions for signs of trouble. The trouble could show up as a decline in the amount of sea ice in the Arctic or crumbling ice sheets in the Antarctic -- both could prove to be the early warning signs of harmful global warming, like the proverbial canary in a coal mine.